PHYSICAL PROPERTIES OF FATTY ACIDS 103 



for the several polymorphic forms. Many different dilatometers have 

 been suggested, including those of Normann,^^^ Coffey and Spannuth,^*^ 

 and of Bailey and co-workers. ^**~^^^ 



b. Refractive Index. The refractive index is another widely used con- 

 stant, since it can be readily determined with great accuracy. It has been 

 employed to ascertain the purity of the fatty acids. 



The refractive index gives a measure of the deviation when a beam of 

 light passes from air into a liquid. The law of refraction was established 

 as early as 1621 by Willebrod Snell, who demonstrated that the ratio of the 

 sines of the angles of incidence and of refraction is a constant at the bound- 

 ary between two media. ^ The refractive index, n, was shown to be the 

 ratio of the velocity of light in a vacuum to that of the light in a liquid. 

 It can be calculated by the equation: 



n = sine i/sine r 



The extent of refraction is governed by the interaction of electrostatic and 

 electromagnetic forces set up by the atoms in the molecules of the liquid 

 with the corresponding properties of the traversing light waves. Since it 

 depends upon intermolecular attraction, it is related to molecular volume 

 and internal pressure. 



The importance of the refractive index was evident from the pioneer work 

 of Eijkman^*^ ^nd of Scheij^^^ on the naturally occurring acids and of 

 Falk^^^ on butyric acid. More complete studies have been made by 

 Eisenlohr and Wohlisch^^^ and by Waterman and Bertram. ^^^ The former 

 workers consider that the product of the refractive index and the molecular 

 weight is additive, while the latter investigators believe that the increments 

 of this value follow two distinct patterns, the signs for which change be- 

 tween the Cii and C12 acids. Verkade and Coops^^^ have challenged the 

 latter view and ascribe the results to the experimental conditions under 

 which the determinations were made. Data on the refractive indices of 

 the common saturated fatty acids are summarized in Table 36. Wyman 

 and Barkenbus^^* have determined the refractive index on some methyl 

 esters of the fatty acids, and this work has been extended by Mattil and 



3« W. Normann, Chem. Umschau, 38, 17-22 (1931); Chem. AbsL, 25, 3858 (1931). 



'" C. A. Coffev and H. T. Spannuth, Oil & Soap, 17, 41^2 (1940). 



'"8 A. E. Bailev and E. A. Kraemer, Oil & Soap, 21, 251-253 (1944). 



3« E. A. Kraemer and A. E. Bailey, Oil & Soap, 21, 254-256 (1944). 



36" A. E. Bailey and W. S. Singleton, Oil & Soap, 22, 265-271 (1945). 



35' W. S. Singleton and A. E. Bailey, Oil & Soap, 22, 295-299 (1945). 



3*2 J. F. Eijknmn, Rec. trav. chim., 'l2, 157-197; 268-285 (1893). 



^^ L. T. C. Scheij, Rec. trav. chim., 18, 169-210 (1899). 



^' K. G. Falk, /. Am. Chem. Soc, 31, 86-107 (1909). 



355 F. Eisenlohr and E. Wohlisch, Ber., 53, 1746-1766 (1920). 



35« H. I. Waterman and S. H. Bertram, Rec. trav. chim., 46, 699-702 (1927). 



•«' P. E. Verkade and J. Coops, Rec. trav. chiin., 47, 45-51 (1928). 



358 F. W. Wyman and C. Barkenbus, Iml. Eng. Chem., Anal Ed., 12, 658-661 (1940). 



